Process for the production of a high methane content town gas

ABSTRACT

An improved process for the production of a methane-rich town gas is described wherein the hot gaseous product of the partial combustion of a carbonaceous fuel is passed into a carbureting zone at a temperature of from about 650* to about 1100*C and therein enriched with a volatile hydrocarbon carburant which on evaporation into the gaseous product of partial combustion is thermally cracked to form lower molecular weight hydrocarbons including methane gas. In this process the methane content of the carbureted gas is substantially increased and the formation of soot and coke during cracking of the hydrocarbon fraction in the carburetor is suppressed by mixing the hot gaseous product of partial combustion with a cool hydrogen-containing gas to cool the hot gas product to a temperature in the range described, prior to carburetion with the volatile hydrocarbon carburant.

United States Patent [191 Ter Haar I PROCESS FOR THE PRODUCTION OF A HIGH METHANE CONTENT TOWN GAS [75] Inventor: Leonard W. Ter Haar, The Hague,

Netherlands [73] Assignee: Shell Oil Company, Houston, Tex.

[22] Filed: Feb. 22, 1974 211 Appl. No.: 444,930

[30] Foreign Application Priority Data Mar. 19, 1973 United Kingdom 13114/73 [52] US. Cl 48/212; 48/219 [51] Int. Cl. .4 C10K 3/06 [58] Field of Search .1 48/199 R, 199 FM, 197 R, 48/197 FM, 205, 212, 213, 215, 219, 96,

OTHER PUBLICATIONS Institute of Gas Technology, Chicago Research Bulle PA RT/A L COMBUSTION REA C TOR 1 Sept. 9, 1975 tin, No. 31, Pipeline Gas From Coal by Methanation of Synthesis Gas, Page 5,

Primary Examiner-S. Leon Bashore Assistant ExaminerGe0rge C. Yeung Attorney, Agent, or FirmA. A, Jecminek [57] ABSTRACT An improved process for the production of a methanerich town gas is described wherein the hot gaseous product of the partial combustion of a carbonaceous fuel is passed into a carbureting zone at a temperature of from about 650 to about 1100C and therein enriched with a volatile hydrocarbon carburant which on evaporation into the gaseous product of partial combustion is thermally cracked to form lower molecular weight hydrocarbons including methane gas, In this process the methane content of the carbureted gas is substantially increased and the formation of soot and coke during cracking of the hydrocarbon fraction in the carburetor is suppressed by mixing the hot gaseous product of partial combustion with a cool hydrogencontaining gas to cool the hot gas product to a temperature in the range described, prior to carhuretion with the volatile hydrocarbon carburant.

9 Claims, 2 Drawing Figures PMENTEUSEP 91% 3.904.388

PART/AL COMBUSTION CARBURETOR\ REA/(570R 4 I 8 1 PARTIAL COMBUSTION CARBURETOR REACTOR C02 74 REMOVAL WASTE UNIT 10, HEAT BOILER CATALYTIC 12 CO-SHIFT REACTOR PROCESS FOR THE PRODUCTION OF A HIGH METHANE CONTENT TOWN GAS BACKGROUND OF THE INVENTION This invention relates to an improved process for the production of a synthetic natural gas of sufficient calorific value to be useful as town gas. More particularly, this invention is directed to an improved process for production of methanerich town gas by carburetion of the gaseous product of partial combustion with a volatile hydrocarbon carburant in which an effective and economical means is employed to substantially increase the methane content of the product gas while at the same time suppressing the formation of soot and coke which traditionally occurs in such process employing carburetion.

The increasing demand for town gas in the last few years has stimulated interest in its production via partially combusting hydrocarbon fractions and then carbureting the resulting combustion gases. However, the realization of processes utilizing this method has been hampered by the fact that gas having a high methane content, and therefore a high calorific value, is not readily produced.

The gas produced by partially combusting a carbonaceous fuel with an oxygen-containing gas normally has a temperature in the range of l300 to lU() although under certain operating conditions temperatures of as high as l 00C or as low as 1 150C are experienced. In any event, however, the temperature of the gas is too high to allow the gas to be directly carbureted with a hydrocarbon fraction and therefore it is first necessary that it be cooled. This cooling is necessary in view of the fact that if the gas is carbureted at a high temperature there is considerable tendency for the hydrocarbon fraction to form soot rather than the desired methane. The soot is then the source of operating troubles in the carburetor and subsequent plant.

Previously the cooling of the hot gas emanating from the partial combustion reactor has been carried out solely by the injection of water and/or steam. This however has the disadvantage that the high steam partial pressure thereby produced promotes the decomposition of methane present in the gas to carbon monoxide and hydrogen according to the reaction CH H O C0 3H Accordingly, it would be desirable if a method for cooling the hot gaseous product of partial combustion could be devised by which the gas could be effectively and economically cooled to a temperature desired for carburetion without the concomittant decomposition of the desired methane product of carburetion Summary of the Invention It has now been found that a town gas having a high methane content can be produced by carburetion of the gaseous product of the partial combustion of a carbonaceous fuel with a volatile hydrocarbon carburant, if the hot partial combustion product gas is mixed with a cool hydrogen-containing gas to cool the hot gas to a temperature in the range of about 650 to about ll00C prior to carburetion. Accordingly, the instant invention provides a process for the production of a methane-rich town gas which comprises:

a. partial combustion of a carbonaceous fuel to produce a hot gas product having a temperature in the range of about 1 to about l700C',

b. cooling the hot gas product of partial combustion to a temperature in the range of about 650 to about 1 [00C by mixing said hot gas product with a cool hydrogen-containing gas;

c. carbureting the cooled gas with a volatile hydrocarbon carburant to produce a gaseous product having a high methane content.

The mixing of the hot gas product of partial combustion with the cool hydrogen-containing gas is suitably carried out by injecting a cool hydrogen-containing gas into a stream of hot gas.

Although a relatively small amount of steam is preferably injected together with the hydrogen-containing gas, the injection of high amounts of water and/or steam into the hot combustion gas operates against the desired objective, i.e., the production of a gas having a high methane content. A major advantage of the present invention is that cooling of the hot gas is primarily carried out by mixing with a hydrogen-containing gas whereby decomposition of methane in the manner previously discussed does not occur. A further advantage is that since a high hydrogen partial pressure is pro duced in the gas the formation of soot and coke during cracking of the hydrocarbon fraction in the carburetor is suppressed.

THE DRAWINGS The invention will be described in greater detail with reference to the accompanying drawings. These drawings are diagrammatic illustrations of the process flow scheme contemplated by this invention and as such are intended to be illustrative rather than limiting on its scope.

FIG. 1 is a diagrammatic representation of the process of the invention wherein hydrogen-containing gas is supplied from a source external to the process.

FIG. 2 is also a diagrammatic representation of an alternative embodiment of the process according to the invention wherein the hydrogen-containing gas is generated internally by certain modifications on the basic process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first step of the process according to the invention involves a typical partial combustion procedure wherein a conventional carbonaceous fuel source, such as for example, heavy fuel oil, petroleum distillates or residues or liquid fuel containing process soot is combined with less than the theoretical amount of oxygen present in an oxygen-containing gas at a temperatures in excess of l00()C, e.g., l l50l 700C, to yield a gaseous product containing as principal components, hydrogen and carbon monoxide. Especially preferred fuel sources are the heavy oils and tars obtained from, for example, tar sands or oil shale. Since this step of the process is wholly conventional and quite well known by those skilled in the art it need not be detailed further herein.

The composition of the cool hydrogen-containing gas which is mixed with the hot gas emanating from the partial combustion reaction may vary widely. Accordingly, it may comprise substantially pure hydrogen or it may contain oxides of carbon, nitrogen, water vapor and/or hydrocarbons. Water gas or producer gas are suitably used as are also tail gas from hydroprocesses and refinery gas. In order to prevent a significant amount of soot and coke being formed as a result of the mixing of the hot partial combustion product gas with the cool hydrogen-containing gas it is preferable that no hydrocarbons heavier than butane are present in the said gas. The reason for this is that heavier hydrocarbons than butane, such as. for example. naphtha, tend to crack at high temperature to form soot and coke. This tendency is not shown to a significant extent by butane and lighter hydrocarbons.

As was indicated above, a suitable method of mixing or combining the cool hydrogen-containing gas with the hot partial combustion product gas is to inject a stream of cool hydrogen-containing gas into the hot gas stream emanating from the partial combustion reactor. The volume of cool hydrogen-eontaining gas which is required to be injected into the hot gas depends mainly on the temperature of the cool hydrogen-containing gas and the temperature to which the hot gas is required to be cooled. it also depends on whether or not it is desired from the point of view of hydrogen optimization to inject a low amount of steam into the hot gas since this will have a cooling effect on the hot gas. Since the temperature of the cool hydrogen-containing gas may vary widely, ranging preferably from ambient temperature to 500C, and the temperature to which the hot gas is to be cooled may also vary widely. e.g., from about 650 to about 1 100C. it is clear that the volume ratio of cool hydrogen-containing gas to hot gas varies over a wide range. A preferred volume ratio of cool hydrogen-containing gas to hot gas used in the process according to the present invention lies between 1 2 and 5 i. In any case. it is preferred that the hot gas be cooled to a temperature in the range of from about 650 to about 950C.

The injection of the cool hydrogen-containing gas may take place in the partial combustion reactor. In this case it is preferably injected at a point near the hot gas exit where the partial combustion of the reactants is substantially completed. Most preferably, however. it is injected into the hot gas as it passes from the reac* tor to the carburetor. ln this way. better mixing of the hot gas and cool hydrogen-containing gas is achieved and also the injection of the cool hydrogen-containing gas does not interfere with the partial combustion reaction. Injection of part of the cool hydrogen-containing gas into the partial combustion reactor and the remaining part into the gas passing from the reactor to the car buretor may also be carried out if desired.

According to a preferred embodiment of the inven tion part of the hot gas leaving the partial combustion reactor is cooled, the cooled gas is subjected to a catalytic CO-shift and the cool hydrogen-containing gas thus produced is injected into the remaining part of the hot gas. In this way hydrogen'containing gas from an external source is not required for cooling the hot gas. This is of advantage either where hydrogen-containing gas from an external source is not available or where it can only be obtained at great expense. The cooling of the part of the gas to be subjected to the catalytic CO- shift is suitably carried out in a waste heat boiler in which steam is raised. The temperature to which the hot gas is cooled preferably lies in the range of 175C to 600C. In the catalytic CO-shift the hydrogen content of the gas is increased by the reaction of steam and carbon monoxide to give hydrogen and carbon dioxide. The cool hydrogen-containing gas thus produced is then injected into the hot gas. Preferably the carbon dioxide is removed from the gas leaving the CO-shift before the latter is injected into the hot gas. This is suitably carried out by a hot carbonate wash. The advantages of removing the carbon dioxide are that the gas which is injected into the hot gas has a higher hydrogen content and that there is no carbon dioxide present in the gas mixture which could react with hydrogen in the hot gas to form steam and carbon monoxide and thus reverse the CO-shift which has already been carried out.

The temperature of the cool hydrogen-containing gas which is injected into the hot gas according to this specific embodiment of the invention lies in a broad range. As in the case of the general embodiment of the invention which has already been discussed this temperature preferably lies between ambient temperature and 500C. If the cool hydrogen-containing gas is injected into the hot gas directly from the CO-shift then its temperature normally lies between 250 and 450C. If a hot carbonate wash is used then the temperature of the gas injected normally lies between 50 and l50C. Of course, if the temperature of the gas to be injected is desired to be increased or decreased then this may be easily carried out by heating or cooling the gas leaving the CO-shift or the hot carbonate wash. In this connec tion the hydrogen partial pressure which is desired to be created in the gas mixture passing to the carburetor is of importance. If a high hydrogen partial pressure is required then it may be that not sufficient cool hydrogen-containing gas can be injected into the hot gas to achieve this high hydrogen partial pressure while still remaining within the desired temperature range for carbureting the gas mixture. This may, for instance. occur if the cool hydrogen-containing gas has a temperature of 50C in which case it may not be possible to inject sufficient cool hydrogen-containing gas to achieve the desired hydrogen partial pressure in the gas mixture produced without cooling the gas mixture below the desired carbureting temperature. In such a case it is desirable to heat the hydrogen-containing gas before it is injected into the hot gas. This suitably is carried out by one or more heat exchangers.

As in the case of the general embodiment of the present invention, the volume ratio of cool hydrogencontaining gas to hot gas varies over a broad range. Preferably it lies between 1 l0 and 5 i. From an economic and practical viewpoint it more preferably lies between 1 l0 and 3: l.

If the carbureted gas produced according to this specific embodiment of the invention is to be catalytieally methanated then it is desirable to operate the process such that the ratio of hydrogen to carbon monoxide in the carbureted gas is 3 l, i.e., the stoichiometric ratio for the production of methane from hydrogen and carbon monoxide according to Sabatiers reaction.

This is suitably carried out according to the present embodiment of the invention by accurately controlling the amount of hot gas which is subjected to the CO- shift, the temperature to which the hot gas is cooled before being carbureted and the amount of hydrocarbon which is injected into the carburetor. ln this way no additional (TO-shift is required to be carried out on the gas leaving the carburetor.

As has been previously mentioned the cooled gas is preferably carbureted within a temperature range of 650 to 950C. In this way soot and coke formation in the carburetor does not occur to a significant extent. and the heating value is promoted. In order to success fully suppress the formation of soot and coke, however, the hydrogen partial pressure in the cooled gas should be above 4 atmospheres and preferably substantially above that figure. An attractive feature of the process according to the present invention is that a hydrogen partial pressure of above 4 atmospheres is always achieved in the cooled gas. Preferably the partial pressure of hydrogen in the cooled gas lies between 7 and atmospheres although higher hydrogen partial pressures can be achieved. Accordingly, soot and coke formation does not occur to a significant extent in the car buretor. Another advantage of the high hydrogen par tial pressures achieved according to the present invention is that the extent to which the gas can be carbureted without soot and coke forming is increased. Ac cordingly, the carbureted gas produced has a high calorific value.

Any suitable hydrocarbon fraction may be injected into the cooled gas in the carburetor. A very suitable hydrocarbon fraction is a distillate fraction boiling below 350C. Gasoline fractions and kerosine fractions are well suited as hydrocarbon fractions.

Since the carbureting of the cooled gas in the carburetor is slightly exothermic it may be desirable in certain cases to stabilize the temperature of the gas by injecting small amounts of cool hydrogen-containing gas into the carburetor. This may be done, if desired. at a number of points within the carburetor. It may also be desirable to inject the hydrocarbon fraction into the carburetor at a number of points. In this case cool hydrogen-containing gas may suitable be injected into the carburetor in between the hydrocarbon injection points. A preferred form of carburetor for this operation is a carburetor in the form of a U-pipe.

The methane-rich gas produced according to the present invention preferably contains more than 25% by volume of methane on a carbon dioxide free basis. However, methane-rich gas having a substantially higher methane content than this is capable of being produced according to the process of the present invention. For example, methane contents of 35% and up to 60% are readily achieved. The methane-rich gas also contains a certain amount of aromatics. These are ad vantageously removed before the gas is further treated and the removed aromatics may suitably be recycled to the reactor to be combusted.

The invention will now be further elucidated with ref crencc to the drawings in which accessories such as valves, pumps, control instruments and the like are not shown.

FIG. I is a diagrammatic representation of a process for preparing a methane-rich gas in which the combustion gas leaving the reactor is cooled by the injection of a hydrogcn containing gas from an external source. FIG. 2 shows another embodiment of the present invention in which part of the hot gas from the reactor is cooled. subjected to a CO-shift and subsequent CO removal and then injected into the remaining part of the hot gas.

With regard to FIG. 1 a fuel is introduced via a line I and oxygen is introduced via a line 2 into a partial combustion reactor 3. Hot gas leaves the reactor via a line 4 and is cooled by the injection of a cool hydrogen containing gas into line 4 via a line 5. The cooled gas then passes to a carburetor 6 into which a hydrocarbon fraction is injected via a line 7. Methane-rich gas leaves the carburetor via a line 8.

Referring to FIG. 2, the integers l to 8 represent the same components of the process as in FIG. 1. Cooling of the hot gas is however carried out by bleeding off a part of the hot gas via a line 9 and cooling it in a waste heat boiler 10. The cooled gas then passes via a line 1 l to a catalytic CO-shifl reactor 12 in which a hydrogen and carbon dioxide-containing gas is produced. This gas leaves the catalytic CO-shift reactor via a line 13 and enters a carbon dioxide removal unit 14 in which the carbon dioxide is removed by a hot carbonate wash. The cooled hydrogen-containing gas thus produced is injected via the line 5 into the remaining part of the hot gas in the line 4.

The following examples will further elucidate the in vention.

EXAMPLE I According to the Invention In a first run 54 kilograms per hour of a fuel oil huving a specific gravity of 097 were partially combusted at a pressure of 60 atmospheres in a reactor to which 40.5 m" (STP) per hour of oxygen were supplied. The combustion gas which left the reactor had a tempera ture of I375C and the following percentage composi tion:

H 40,33 J? by volume H 9 I2 27 1 by volume Co 40.48 by volume C0 4 .13 vi by volume CH, 0.53 "i by volume N Ar H 8 2.0K: Z by volume 128 m (STP) per hour of a hydrogen-containing gas having a temperature of 50C and 9.7 kg steam having a temperature of 3 l0C were injected into the combustion gas to give a cooled gas having a temperature of 850C. The composition of the hydrogcn-containing gas was:

H 87.90 '4 by volume CH 0.70 J by volume CO 9.80 '1 by volume N Ar H 5 l (10 71 by volume H: 23.0 I; by volume (H 49.0 J by volume (0 25.6 h by volume CO N Ar 2.4 I; by volume EXAMPLE II Not according to the Invention In a second run the abovementioned process was carried out under exactly the same conditions as given above except that cooling of the hot gas to a tempera ture of 850C was carried out by the injection of 36 ki lograms per hour of water. The cooled gas which had a hydrogen partial pressure of 23 atmospheres was then passed to a carburetor into which a hydrocarbon frac tion boiling in the range of l40 to l60C was injected at a rate of 35 kg per hour. After removal of water. H S. soot, aromatics and part of the CO this operation resulted in 160 m"(NTP) carbureted gas having a caloric value of 5080 kcal/m (NTP) and a composition of:

H, 305 91' by volume CH 33.0 a' by volume CO 34.0 P; by volume C(). N;- Ar 2.5 '7: by volume From the results of these two runs it is apparent that the use of a cool hydrogen-containing gas for cooling the hot gas is very much better from the point of view of producing a gas having a high methane content, ie, a gas having a high calorific value. than the use of water for cooling the hot gas.

What is claimed is:

l. A process for the production of a methane-rich town gas which comprises:

a. partially combusting a carbonaceous fuel to produce a hot gas product having a temperature in the range of about 1 [50 to about l700C;

b. cooling the hot gas product of partial combustion to a temperature in the range of about 650 to about 1 100C by mixing said hot gas product with a cool hydrogen-containing gas to produce a cooled gas having a partial pressure of hydrogen above 4 atmospheres;

c. carbureting the cooled gas with a volatile hydrocarbon carburant to produce a gaseous product having a high methane content.

2. The process according to claim 1, wherein the mixture of the hot gas product of partial combustion with the cool hydrogen-containing gas is carried out by in jecting a cool hydrogencontaining gas into a stream of the hot gas.

3. The process according to claim 2, wherein the hot gas product of partial combustion is cooled to a temperature in the range of about 650 to about 950C by injection of the cool hydrogen-containing gas.

4. The process according to claim 3, wherein the volume ratio of the cool hydrogen-containing gas to the hot gas product of partial combustion lies between I l0 and 5 l.

5. The process according to claim 4, wherein the par tial pressure of hydrogen in the cooled gas lies between 7 and 15 atmospheres.

6. The process according to claim 1, wherein a portion of the hot gas product of partial combustion is cooled and subject to a catalytic CO-shift reaction to produce a cool hydrogen-containing gas which is mixed with the remaining hot gas product of partial combus tion to provide at least a portion of the cooling required to cool the hot gas.

7. The process according to claim 6, wherein the mixing of the hot gas product of partial combustion with the cool hydrogen-containing gas is carried out by injecting the cool hydrogen-containing gas into a stream of the hot gas.

8. The process according to claim 7, wherein the carbon dioxide is removed from the cool hydrogencontaining gas product of the (O-shift reaction prior to its being injected into the remaining part of the hot gas.

9. The process according to claim 1 wherein the volatile hydrocarbon carburant is a hydrocarbon fraction boiling below 350C. 

1. A PROCESS FOR THE PRODUCTION OF A METHANE-RICH TOWN GAS WHICH COMPRISES: A. PARTICALLY COMBUSTING A CARBONACEOUS FUEL TO PRODUCE A HOT GAS PRODUCT HAVING A TEMPERATURE IN THE RANGE OF ABOUT 1150* TO ABOUT 1700*C. B. COOLING THE HOT GAS PRODUCT OF PARTIAL COMBUSTION TO A TEMPERATURE IN THE RANGE OF ABOUT 650* TO ABOUT 1100*C BY MIXING SAID HOT GAS PRODUCT WITH A COOL HYDROGEN-CONTAINING GAS TO PRODUCE A COOLED GAS HAVING A PARTIAL PRESSURE OF HYDROGEN ABOVE 4 ATMOSPHERES,
 2. The process according to claim 1, wherein the mixture of the hot gas product of partial combustion with the cool hydrogen-containing gas is carried out by injecting a cool hydrogen-containing gas into a stream of the hot gas.
 3. The process according to claim 2, wherein the hot gas product of partial combustion is cooled to a temperature in the range of about 650* to about 950*C by injection of the cool hydrogen-containing gas.
 4. The process according to claim 3, wherein the volume ratio of the cool hydrogen-containing gas to the hot gas product of partial combustion lies between 1 : 10 and 5 :
 1. 5. The process according to claim 4, wherein the partial pressure of hydrogen in the cooled gas lies between 7 and 15 atmospheres.
 6. The process according to claim 1, wherein a portion of the hot gas product of partial combustion is cooled and subject to a catalytic CO-shift reaction to produce a cool hydrogen-containing gas which is mixed with the remaining hot gas product of partial combustion to provide at least a portion of the cooling required to cool the hot gas.
 7. The process according to claim 6, wherein the mixing of the hot gas product of partial combustion with the cool hydrogen-containing gas is carried out by injecting the cool hydrogen-containing gas into a stream of the hot gas.
 8. The process according to claim 7, wherein the carbon dioxide is removed from the cool hydrogen-containing gas product of the CO-shift reaction prior to its being injected into the remaining part of the hot gas.
 9. The process according to claim 1 wherein the volatile hydrocarbon carburant is a hydrocarbon fraction boiling below 350*C. 